I am looking for a design to build a control for my really old hot tub. Currently I switch it manually monitoring the temp but due to inattention, frequently over heat it, which causes the wife to overheat. I would like to have two modes, a "maintianing" function which would not allow the temp to drop below a certian level, say 65 F, and an "operating" mode which would bring the temp up to about 103 F and hold it there plus or minus a half degree. I would like to be able to change those values as the season requires. Am currently using a cheap inside outside temp monitor with the probe (thermistor) mounted in a wet well in the out line from the heater. I would like to incorporate a temp display in the new design to simplify the set up. Any help or links would be appreciated.

Jeff

 

There may be ready made controllers on the market. Have you checked?

The microcontroller guys will tell you it's a relatively simple project and I posted one such concept below. If you're not interested in learning a new language, so to speak, but you're pretty good at electronics then perhaps someone on the Forum could throw together the program and 'burn' it onto a microcontroller for you.

I did a demo' using the Dallas/Maxim DS18B20 "one-wire" temperature chip on this Forum.Microchip thread last year; DS18B20 Temperature Readings?

Good luck with your project.

Mike

Attached Images

Temperature Controller.PNG (54.6 KB, 19 views)

 

Perhaps an LCD would be better in that hot tubs are often used outdoors. Easier to read.

I use dallas 1 wire temperature sensors to keep track of temperature in my ponds. To make the water proof I stick them into a soda straw filled with epoxy. Learned that from P.H. Anderson.

You may be able to find some GP controller to do this. All it needs is a 1 wire driver and an output driving a relay of sufficient size.

Above all be sure to power the controller and the rest of the tub setup off a GFI circuit.

Historical Note:

15 or so years ago when gas was much cheaper then electricity I knew a guy who build a Z80 based controller for his hot tub. He used a natural gas fired water heater to provide the heat.

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I'd encourage you to add a high limit to the mix of things you want to do - for safety more than anything else. I think current requirements in US are to include a safety. I think much of the reason is because you can get into the tub at a moderate temperature and possibly not sense that you are overheating if the heating controls were to run wild. Sure, at some point you'd figure it out but not until you've dangerously stressed body systems.

Consider this too - that there is probably some constant loss that you need to make up - this could be a separate heating element that is on full time or nearly so. Then you might apply controls to a second heating element. That could improve control or at least reduce the current levels you need to handle.

They do make low cost commercial industrial controllers that will directly drive heating elements. You might look to this if you do not find what you need commercially.

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